1. Field of the Invention
This invention relates to a permanent magnet rotating machine such as a servo motor, and more particularly to a permanent magnet rotating machine of small size and high output power in which the cogging torque is reduced.
2. Description of the Related Art
In the typical constitution of a permanent magnet rotating machine, a rotor is disposed inside a stator. The stator includes a plurality of magnetic poles with a plurality of stator windings on the inner circumference of a stator iron core that is almost cylindrical in shape and a plurality of convex poles protruding inwardly. The rotor has a rotor iron core disposed to be rotatable around a center of the stator as a central axis of rotation. A permanent magnet is located on the surface or inside of the rotor iron core, and magnetized so that N poles and S poles are arranged alternately in a circumferential (rotational) direction. In this rotating machine, the stator windings are appropriately energized to generate a rotational magnetic field so that the rotor is rotated around the central axis of rotation.
In the above permanent magnet rotating machine, a rotational torque ripple called a cogging torque occurs even in the absence of a load. The cogging torque produces a vibration or noise, or degrades the control performance of the rotating electric machine.
To reduce this cogging torque, it is well known to provide a skew at the boundary lines between magnetic poles of the permanent magnet. Generally, the interpole between N pole and S pole of the permanent magnet is a linear space oblique to the central axis of rotation. A theoretical skew angle α (mechanical angle) for reducing the cogging angle most greatly isα=360/(smallest integer of which both the number of windings and the number of magnetic poles on the stator side are factors)[deg](refer to JP-A-2000-308286).
If this theoretical skew angle is represented in electrical angle in terms of the number of magnetic poles (poles) of the rotor and the number of magnetic poles (slots) of the stator, the theoretical skew angle θs for reducing the cogging torque most greatly is given byθs=180×(number of magnetic poles of the rotor)/(smallest integer of which both the number of windings of the rotor and the number of magnetic poles of the stator are factors)[deg]  (1)